Known-Plaintext Attack - Computer Definition

The
simplest means to “brute-force” a key using a sample of both the encrypted
message and the original plaintext.
A known-plaintext attack is a cryptographic
attack in which an individual has the plaintext and its encrypted version (ciphertext), thereby allowing him or
her to use both to reveal further secret information—such as the secret key.
Encrypted archived ZIP files are said to be prone to known-plaintext attacks
because using software available on the Internet, crackers are able to determine
the key needed to decrypt the archived files.